Die-to-wafer bonding is a commonly used bonding method, wherein semiconductor dies sawed from wafers are bonded to semiconductor chips in un-sawed wafers. With the die-to-wafer bonding, known-good-dies may be selected and bonded to wafers. The yield is thus improved over that of wafer-to-wafer bonding.
The conventional die-to-wafer bonding methods suffer from drawbacks. A wafer may include many chips (referred to as bottom chips hereinafter), sometimes as many as one thousand chips or more. Accordingly, more than one thousand top dies need to be bonded to the bottom chips one by one. This may take up to two hours. During the entire period of the bonding process, the wafer needs to be heated. The temperature of the wafer, however, is difficult to control. If the temperature is high, the copper bumps in the bottom chips suffer from serious oxidation, and the flux applied on the bottom chips may be baked/evaporated. On the other hand, if the temperature is low, cold joints may be formed. As such, the yield of the die-to-wafer bonding is adversely affected.